(1) Field of the Invention
This invention relates to a method for the controlling idling speed of of an internal combustion engine, and more particularly to such a method which effects feedback control of the idling speed by controlling the amount of inlet air to the internal combustion engine by means of a control valve disposed in a bypass interconnecting the upstream and downstream sides of a throttle valve inserted in an intake passage of the internal combustion engine.
(2) Description of the Prior Art
It has been customary to control the idling speed of an internal combustion engine through control of the amount of inlet air to the internal combustion engine by means of a control valve disposed in a bypass interconnecting the upstream and downstream sides of a throttle valve during a so-called idle operation or low-load operation, in which a throttle valve in an intake passage is kept in a substantially completely closed state.
In an automobile provided with an automatic transmission of fluid coupling, the load of the automatic transmission is exerted on the internal combustion engine while the automatic transmission is in its in-gear state, i.e. while the position of the selector is in its drive (D) range. It has been customary, therefore, to prevent the idling speed from dropping while the automatic transmission is in the drive (D) range by adjusting the inlet air control valve in its opening direction thereby increasing the amount of inlet air and enabling the mixture supplied into the engine to be increased.
It is generally known that in an internal combustion engine of the electronically controlled fuel injection type, an increase in the amount of inlet air results in a proportional increase in the amount of fuel to be injected and, consequently, in an increase in the amount of mixture.
The degree of opening of the control valve is controlled in a closed loop during an idling operation, i.e. while the throttle valve is substantially completely closed and the speed of engine rotations is in a prescribed range of idling rotations. An exciting current supplied to a solenoid proportionately controlling an opening angle of the control valve is fixed on the basis of a solenoid current command Icmd which is obtained in accordance with the following formula (1): EQU Icmd=Ifb (n)+Iat (1)
wherein Ifb(n) denotes a PID feedback control term (basic control term) for effecting proportional (P term), integral control(I term), and derivative(D term) actions based on a deviation of the actual number of engine rotations Ne from the target number of idling rotations Nrefo and Iat denotes a correction term which is a constant Iato that is applicable while the automatic transmission is in D range.
As known well, the automatic transmission is provided with a pump impeller of a torque converter connected directly to the engine and a turbine runner connected directly to the output shaft, and the slip rate of the automatic transmission is fixed by the ratio of the rotational speed of the impeller and runner. In other words, the ratio between the speed of engine rotations and the speed of the automobile determines the slip rate.
During an idling operation, the slip rate reaches its maximum value when the automatic transmission is in the D range and the automobile is kept stopped by putting on the brakes.
When the automobile is travelling in a creep state or in the state of engine braking, the slip rate is lower than when the automobile is kept stopped by putting on its brakes. As a result, in such an operating state the external load on the engine generated by the automatic transmission (hereinafter referred to as "AT load") is lowered, too.
The addition correction term Iat of the formula (1) mentioned above is generally fixed at a prescribed value Iato which permits correction of the AT load enough to prevent a decrease in the idling speed of the automobile when the engine is kept in an idle operation after warming of the engine has been completed and the speed of the automobile is still zero.
When the AT load is small as described above, or the automobile is travelling in the creep state or in the state of engine braking, the magnitude of the addition correction term Iat turns out to be too large for the actual magnitude of AT load. This trend becomes conspicuous particularly when the speed of engine rotations approaches the lower limit of the prescribed range of speed of idling rotations.
As a result, the magnitude of the feedback control term Ifb(n) for adjustment to the target number of idling rotations, Nrefo, is decreased.
Where the magnitude of the feedback control term Ifb(n) is set at a small level as described above, a sudden application of the brakes during the travel of the automobile in the creep state or in the state of engine deceleration results in a sharp increase in the AT load. There ensues a disadvantage that the decrease in the speed of engine rotations due to the increase in the AT load can no longer be corrected by the feedback control term Ifb(n) and the number of engine rotations is greatly decreased or the engine stalls .
The magnitude of the feedback control term Ifb(n) is also decreased when the state of engine braking is started while the automobile is travelling on a descending slope to lower the speed of the automobile from the state of highspeed operation until the number of engine rotations falls within the range of numbers of idling rotations and the operation of the control valve is shifted to the feedback control mode. When the vehicle brakes are suddenly applied in this case as in the case mentioned above, the number of engine rotations is greatly decreased or the engine stall.
The PID coefficient (proportional, integral, and derivative control action gain) in the feedback control term Ifb(n) in the formula (1) is generally set at a small level. As the result, the feedback control by this term Ifb(n) is generally carried out slowly. This is because the stability of the stationary idle operation is impaired when the control gain is increased to increase the magnitude of feedback control.